1. Field of the invention
The present invention relates to a method of and a device for detecting the focal point in an optical pickup.
2. Description of the Prior Art
Optical pickups direct the laser beam from a laser beam source through an objective lens onto an optical information storage medium such as an optical memory disk for reading the information stored on the information storage medium or writing information on the information storage medium. For either reading or writing the information, the laser beam has to be focused by the objective lens accurately on the information storage medium.
For accurately focusing the laser beam, it is necessary to detect whether the laser beam is focused by the objective lens on the information storage medium. Various methods have been known as focal point detecting methods. One of such known focal point detecting methods is known as a so-called knife-edge focal point detecting method.
FIG. 5 of the accompanying drawings schematically illustrates an optical pickup employing a knife-edge focal point detecting method. The optical pickup includes a laser beam source 1 comprising a semiconductor laser for emitting a laser beam, a coupling lens 2, a polarization beam splitter 3, a quarter-wave plate 4, an objective lens 5 for focusing the laser beam onto an information storage medium 6, a convergent lens system 7, a first detector 8 for detecting a deviation of the laser beam from a desired track, and a second detector 9 for detecting the focal point.
The laser beam emitted from the semiconductor laser 1 is converted by the coupling lens 2 into parallel rays which are then reflected to the left (as shown) by the polarization beam splitter 3. The laser beam from the polarization beam splitter 3 passes through the quarter-wave plate 4 and the objective lens 5, which focuses the laser beam as a small light spot onto the information storage medium 6.
The information storage medium 6 is generally in the form of a disk bearing recording tracks arranged as concentric circular tracks or in a spiral pattern. The information storage medium 6 will hereinafter be referred to as a disk 6.
As the disk 6 is rotated about its own axis, the laser beam falling on a recording track on the disk 6 is reflected thereby and falls rightwardly on the objective lens 5. The reflected laser beam passes through the objective lens 5 and the quarter-wave plate 4 and falls rightwardly on the polarization beam splitter 3. Since the laser beam passes twice through the quarter-wave plate 4, the plane of polarization thereof has rotated 90.degree., allowing the laser beam to pass linearly to the right through the polarization beam splitter 3 into the focusing lens system 7. After the laser beam has passed through the convergent lens system 7, it is converged thereby toward the second detector 9. The objective lens 5 and the convergent lens system 7 have a common optical axis AX.
As shown in FIG. 6, the second detector 9 has two light detecting areas A, B divided by an intermediate line extending perpendicularly to the sheet of FIG. 5 with the light detecting surfaces lying perpendicularly to the optical axis AX.
The first detector 8 has a straight edge (knife edge) extending perpendicularly to the sheet of FIG. 5. The first detector 8 is positioned between the covergent lens system 7 and the second detector 9.
As illustrated in FIG. 7, the first detector 8 comprises two light detecting areas C, D for detecting 50% of the luminous flux of the laser beam converged by the convergent lens system 7 to block 50% of the laser beam directed toward the second detector 9.
In FIG. 5, the laser beam is properly focused by the objective lens 5 onto the disk 6. At this time, the laser beam reflected by the disk 6 is converted by the objective lens 5 to parallel rays which are passed through and converged by the convergent lens system 7 to the focal point thereof.
Experiments indicated that, regardless of the presence of the first detector 8, the laser beam is converged by the convergent lens system 7 to the focal point thereof as a light spot having a diameter of about 100 micrometers.
Under this condition, photoelectrically converted signals SA, SB are produced respectively from the light detecting areas A, B of the second detector 9, and the difference (SA-SB) is employed as an focal point error signal. The second detector 9 is positionally adjusted with respect to the light spot so that the focal point error signal will be eliminated.
When the laser beam is correctly focused on the disk 6, the focal point error signal (SA-SB) is zero, i.e., the light detecting areas A, B of the second detector 9 detect the same amount of light.
If the disk 6 is shifted from the position of the focal point of the objective lens 5 in a direction away from the objective lens 5, then the laser beam reflected from the disk 6 is converged by the objective lens 5 and also by the convergent lens system 7. The point where the laser beam converges is displaced toward the convergent lens system 7 as shown in FIG. 8, thus increasing the amount of light detected by the light detecting area A of the second light detector 9. Therefore, the focal point error signal (SA-SB) is expressed by (SA-SB)&gt;0.
If the disk 6 is shifted from the focused position toward the objective lens 5, then the laser beam reflected from the disk 6 remains divergent as it passes through the objective lens 5. The point where the laser beam converges through the convergent lens system 7 is displaced rightwardly of the second detector 9 as shown in FIG. 9. The amount of light detected by the light detecting area B of the second light detector 9 is then increased, and the focal point error signal (SA-SB) is represented by (SA-SB)&lt;0.
The optical pickup as a whole or the objective lens 5 only can be displaced along the optical axis AX in a direction dependent on the focal point error signal to eliminate the focal point error signal for thereby focusing the laser beam correctly on the disk 6.
The foregoing process is a summary of the knife-edge focal point detecting method.
Generally, the focal point of the objective lens 5 is detected by the above system, and the objective lens 5 is displaced along the optical axis AX by a servo control system (not shown) so that the focal point of the objective lens 5 will be positioned in a range of .+-.1 micrometer from the recording surrace of the disk 6. In order to avoid the danger of collision between the disk 6 and the objective lens 5, an original position for the objective lens 5 is established at a sufficient distance from the disk 6. At the same time that the disk 6 starts being scanned by the optical pickup, the objective lens 5 is progressively moved toward the disk 6 for focusing the laser beam onto the disk 6. Such a process is referred to as a "focusing process".
As shown in FIG. 10, the focal point error singal (SA-SB) has its voltage level eliminated when the laser beam is focused, and also when the objective lens 5 is spaced about 100 micrometers or more from the focused position in a direction away from the disk 6, or when the objective lens 5 is spaced about 150 micrometers or more from the focused position in a direction toward the disk 6. Where the original position for the objective lens 5 is spaced 500 micrometers from the focused position, and if the focal point were to be detected only by the focal point error signal (SA-SB), then the servo control system would be liable to be locked since the voltage level of the focal point error signal (SA-SB) would be zeroed in conditions, as described above, other than when the laser beam is focused.
One auxiliary way of preventing such a drawback has been to employ an information signal Rf representative of the sum (SA+SB+SC+SD) of the photoelectrically converted signals (SA+SB) from the light detecting areas A, B of the second light detector 9 and the photoelectrically converted signals (SC+SD) from the light detecting areas C, D of the first light detector 8. The information signal Rf is illustrated in FIG. 10 as being half of the acutual value thereof.
The information signal Rf has a certain voltage level when the laser beam is focused on the disk 6. Therefore, the focal point can stably be detected by determining that the focused condition is reached when the focal point error signal (SA-SB) is zero and also the information signal Rf has a predetermined voltage level or higher during the focusing process.
However, the sensitivity of detection of the focal point in the above method is not necessarily high for the following reasons: The laser beam reflected from the disk 6 and passing through the objective lens 5 for the detection of the focal point will be referred to as a focal point detecting beam or flux, which is indicated by FLX in FIG. 7. The focal point detecting beam may also be used for other purposes such as for reading information stored on the disk 6 or for tracking the disk 6.
As shown in FIG. 7, the first light detector 8 blocks 50% of the focal point detecting beam which would otherwise reach the second light detector 9. However, under the focused condition, the laser beam falls on the light detecting area B as well as the light detecting area A due to diffraction.
The focal point detecting beam has an intensity peak in the vicinity of the optical axis AX. The ray which travels on the optical axis of the objective lens 5 will be referred to as an axial ray. The beam or flux in the neighborhood of the axial ray has a strong intensity and falls on both light detecting areas A, B of the second light detector 9 due to diffraction. The beam falling on the second light detector 9 in the neighborhood of the axial ray remains substantially unchanged irrespectively of whether the laser beam is focused on the disk 6 or not.
Light rays which are primarily responsible for substantially changing the focal point error signal (SA-SB) are those spaced from the axial ray.
Therefore, the cause of the detection sensitivity that is not so high is the presence of the beam or flux of the focal point detecting beam which is in the vicinity of the optical axis or axial ray.
The ratio of the difference between the signals from the light detecting areas A, B to the sum of the signals from the light detecting areas A, B is used as the focal point or focus detecting sensitivity. The focal point detecting sensitivity was plotted, as shown in FIG. 11, as the ratio of the focal point detecting beam or flux FLX blocked by the first detector 8 was varied. As a result, it was confirmed that the focal point detecting sensitivity had a peak when 70 to 80% of the flux containing the axial ray was blocked with respect to the second light detector 9, and the focal point detecting sensitivity was effectively increased when the flux was blocked in the range of from 60 to 90%.
Since the flux in the neighborhood of the axial ray is primarily responsible for the reduction of the focal point detecting sensitivity, as described above, it is preferable that the amount of light blocked by the first light detector 8 with respect to the second light detector 9 be at least 1/2 of the total beam which would reach the second light detector 9, or preferably in the range of 60 to 90% of the total beam.
Where the amount of light blocked by the first light detector 8 with respect to the second light detector 9 is in the desired range for an increased focal point detecting sensitivity, only the signals (SC+SD) from the light detecting areas C, D of the first light detector 8 are useful for producing an information signal Rf', without the addition of the signals (SA+SB) from the second light detector 9, as a sufficient amount of light falls on the light detecting areas C, D.
However, the beam detected by the first light detector 8 contains the intensive light in the vicinity of the optical axis. Therefore, the voltage level remains virtually unchanged as shown in FIG. 10 irrespectively of the defocused distance by which the laser beam is out of focus. Consequently, the information signal Rf' cannot be employed in the focusing process. The information signal Rf' is illustrated in FIG. 10 as being 1/5 of the actual value thereof for the sake of brevity.